1. Field of the Invention
Embodiments of the present invention relate generally to video processing and more specifically to a motion-adaptive video de-interlacer.
2. Description of the Related Art
Video frames are typically encoded in an interlaced format comprising a first field and a second field, each field having alternating lines of the video frame and each field being temporally separated. Video images are typically encoded and transmitted in such an interlaced format as a compromise between bandwidth and video image resolution. Since interlaced video frames are displayed using only half the lines of a full video frame, less system bandwidth is required to process and display these types of video frames. However, since the human eye typically cannot resolve a single video field, but rather, blends the first field and the second field, the perceived image has the vertical resolution of both fields combined.
Some types of video screens, such as progressive displays, require the use of de-interlaced video frames instead of interlaced video frames. When using such displays, the video frames encoded in an interlaced format must be de-interlaced prior to display. There are several well-known methods to construct de-interlaced video frames. One such method is commonly referred to as the “weave” method in which both the first field and the second field are displayed simultaneously by weaving together the alternating lines of the two fields. This method produces de-interlaced video frames with relatively good picture quality so long as the images depicted within the video frame are not moving. Another de-interlacing method is commonly referred to as the “bob” method in which a de-interlaced video frame is constructed from a single video field using vertical interpolation to “fill in” the video data missing from the single field. In contrast to the weave method, this method produces de-interlaced video frames with relatively good picture quality when the images depicted within the video frame are moving.
There are several known ways to create a de-interlaced video frame by combining the results of the weave method and the bob method. These “hybrid” techniques attempt to produce video frames higher in quality than those produced using either the weave or bob method alone. One such method uses a difference function to determine the presence or absence of motion within the interlaced video frame on a pixel-by-pixel basis. The difference function compares the value (e.g., luminance values) of a pixel in a current video frame with the value of the same pixel in the previous video frame. If the value of the pixel changes between the video frames, then the assumption is that the pixel is moving. This motion information is then used to select either the weave or the bob method to construct that pixel in the de-interlaced video frame. For example, if the difference function indicates that a pixel is moving, then the bob method is used to construct the pixel in the de-interlaced video frame. If, on the other hand, the difference function indicates that the pixel is not moving, then the weave method is used to construct the pixel in the de-interlaced video frame.
One drawback of this approach is that the difference function does not calculate the magnitude of motion attributed to a pixel. Thus, the bob method is always used whenever any motion is detected. However, as is well-known, the bob method displays an undesirable processing artifact when de-interlacing video images with slow moving objects. To illustrate, consider the top edge of a box slowly moving vertically toward the top of the screen. If the top edge of the box precisely aligns with a video line of the first field, then the corresponding vertically interpolated video frame includes the top edge of the box, but the vertically interpolated video frame corresponding to the second field does not include the top edge of the box. Consequently, as the box moves, the top edge of the box appears to flicker as the top edge aligns with the lines from the first field and the second field.
As the foregoing illustrates, what is needed in the art is a way to de-interlace video frames that produces higher quality frames than current techniques.